LEDs used for area lighting, automotive exterior lighting, medical lighting and television backlighting require a way to dim the LEDs to obtain a desired lighting level and/or average lumen output. LED dimming may be provided with analog linear dimming or pulse width modulation (PWM) dimming. Linear dimming of LEDs is used to reduce/adjust brightness thereof by changing current through the LEDs. Change in current through the LEDs results in a shift of the chromaticity coordinates (change of color temperature). Many applications like retrofit light bulb replacement, automotive lighting, medical lighting or professional illumination systems highly rely on specific color temperatures to meet application specific light requirements or legal regulations. PWM dimming turns on and off (allows current to flow and not flow through the LEDs) at a nominal current necessary to meet specific chromaticity coordinates during the on-time of the LEDs. The on and off frequency for dimming the LEDs has to be high enough to create a seemingly static (constant) light to the human eye.
PWM dimming of constant current sources causes three issues with LEDs: The first issue is high current overshoot as the LED is switched into the circuit (when the current source is turned back on after the dimming off-time). This overshoot shortens the service life of the LED. This effect can particularly be observed in lighting systems where switched-mode DC/DC converters are used as the current source. Control stages of analog switched-mode power converters utilize operational amplifiers as an inverting error amplifier. During the dimming off-time, the feedback signal drops to zero. The analog error amplifier thereupon increases its output voltage (reference voltage to peak current comparators or comparators in PWM generators) to compensate for the instantaneous error. The feedback loop of these amplifiers is closed by a circuit of resistors and capacitors (the compensation filter RC network). This RC network is either connected between the amplifier input and its output (circuit for general purpose operational amplifiers) or between the amplifier output and the circuit ground (circuit for trans-conductance operational amplifiers). When the amplifier output voltage increases to compensate for the instantaneous error during the PWM dimming off-time, the RC network is charged. When the feedback drops to zero, the error is maximal and so the output voltage of the error amplifier will increase up to the saturation point of the circuit. When the PWM dimming signal is turned back on, the error amplifier of the control circuit will force the switched-mode power converter to apply the maximum duty ratio of the switching frequency resulting in a short maximum power output, which will last until the feedback signal has tuned into normal levels of operation and the compensation network has de-saturated. To compensate for this issue, analog circuits are usually added to the error amplifier circuit to apply a fast soft-start ramp. These fast soft-start ramps, however, add a reduced average forward current component to the total LED forward current, causing a shift of chromaticity coordinates (shift in color temperature).
The second issue is a slow forward voltage decay after the current source is switched off that is caused by the discharging output capacitors of the disabled current source. This decay affects the color temperature, which becomes more and more dominant with shorter duty ratios.
The third issue is the physical limitation of minimum dimming PWM duty ratios when systems suffer from slow current slew rates of leading and/or falling edges. The time required to increase the LED forward current up to the nominal level and/or back down to zero limits the minimum on-time required to achieve a certain lumen output. When stable color temperatures are explicit, a minimum period of nominal forward current is required, further increasing the minimum on-time. This becomes an issue in applications when very low on-times and stable color temperatures are mandatory, like automotive exterior lighting, display backlights, medical or restoration lighting applications, and the like.